The present invention relates generally to line-focus systems and, more particularly, to a method for constructing a compensating aperture for a line-focus system which enables accurate measurement of light intensity.
Line-focus systems are utilized in optical scanners and other optical imaging devices. In a line-focus system, a light beam from an illuminated line object is imaged by a lens onto a linear photosensor array which is positioned remotely from the line object. The linear photosensor array is a single-dimension array of photoelements which correspond to small area locations on the line object. These small area locations on the line object are commonly referred to as "picture elements" or "pixels." In response to light from its corresponding pixel location on the line object, each photosensor produces a data signal which is representative of the light intensity which it experiences. All of the photoelement data signals are received and processed by an appropriate data processing system. In an optical scanning device, the illuminated line object of the line-focus system is commonly referred to as a "scan line."
Optical scanners and various components thereof are disclosed in U.S. Pat. No. 4,926,041; U.S. Pat. No. 4,709,144 of Kent J. Vincent; and U.S. Pat. No. 4,870,268, which are all hereby specifically incorporated by reference for all that is disclosed therein.
A problem experienced in most line-focus systems is that the light intensity of the line image produced at the linear photosensor array is not uniformly proportional to the light intensity at the line object. Generally, if a line object is evenly illuminated across its length, the light intensity at the sensor is much brighter in the area corresponding to the center of the line object than in the areas corresponding to the ends of the line object. This effect is quite significant; the intensity at the line image center may be roughly twice the intensity as that at the ends. There are several different optical effects which produce this problem. These optical effects are due mainly to the different distances of the various points on the line object from the central axis of the lens.
It is known in the prior art relating to line-focus systems to utilize an aperture positioned in the light path between a line object and a photosensor to differentially occlude light in the light path extending between the line object and the photosensor. Such "occluding" or "compensating" apertures are designed to occlude a proportionally greater portion of the light at the center of the light path than at the ends so as to make the light intensity at the photosensor uniformly proportional to the light intensity at the line object. A problem with the construction of such occluding apertures is that there is no mathematical formula which may be employed in a straightforward manner to determine aperture shape. As a result, such occluding apertures have, prior to the present invention, been created through trial-and-error empirical methods.